1) Field of the Invention
The present invention relates to an ambient light detector that detects a change in light amount of ambient light, a light source lighting controlling device that controls lighting of a light source using the ambient light detector, and a reader including the ambient light detector and/or the light source lighting controlling device.
2) Description of the Related Art
Conventionally, bar-code readers have been widely used to read bar-codes. The bar-code reader reads a bar-code by irradiating and scanning light flux such as a laser beam onto an article and then condensing and detecting the light flux reflected on the article.
FIG. 25 is a block diagram used for explaining the configuration of a general bar-code reader. Referring to FIG. 25, numeral 101 represents a bar-code printed on a surface of an article. The bar-code 101 includes plural white bars and plural black bars arranged alternately to each other.
Numeral 102 represents an optical system for a bar-code reader. Numeral 103 represents a laser emitting unit, e.g. a laser light source, emitting a laser beam L1.
Numeral 104 represents a scanning mechanism. The scanning mechanism 104 scans the laser beam L1 emitted from the laser emitting unit and sends out of the bar-code reader, and irradiates the scanning beam L2 onto the bar-code 101 while it sends the reflected beam R2 into the photoelectric converting unit (to be described later) in response to the reflection beam R1 reflected on the bar-code 101.
The scanning mechanism 104 is formed of, for example, a polygon mirror driven by a motor and a Galvano-mirror. When the polygon mirror is rotated, the laser beam reflected with the polygon mirror is scanned in a fixed direction and at a fixed rate.
Additionally some scanning mechanisms 104 include a split mirror which produces plural scanning beams L2 from a laser beam scanned with a polygon mirror.
Numeral 105 represents a photoelectric converting unit. The photoelectric converting unit 105 receives the reflected beam R2 irradiated from the scanning mechanism 104 and then outputs an analog electrical signal corresponding to the incident light amount. A photoelectric converting element such as a photo diode is used as the photoelectric converting unit 105.
Numeral 106 represents an A/D converter. The A/D converter 106 converts an analog electrical signal output from the photoelectric converting unit 105 into a digital electrical signal. When the A/D converter 106 converts an electrical signal from the photoelectric converting unit 105 into a digital signal, a binary signal formed of a bar-code 101, which includes a black level signal corresponding to a black bar and a white level signal corresponding to a white bar, is obtained.
Numeral 107 represents a bar-width counter. The barwidth counter 107 counts the time width of a black level signal and the time width of a white level signal output from the A/D converter 106, based on clock signals produced by the clock generator 108. The time width of the black level signal corresponds to the width of a black bar while the time width of the white level signal corresponds to the width of a white bar.
Numeral 109 is a memory. The memory 109 stores a count value output from a bar-width counter, or bar-width data. The CPU 110 receives the count value to demodulate bar-code data.
Formerly, the He-Ne gas laser has been used as a laser light source. Recently small bar-code readers have been demanded. However, since the gas laser is bulky, it has been difficult to bring the bar-code reader to a small size. Hence semiconductor lasers are being preferably used as laser light sources. There is a problem in that although the semiconductor laser, smaller than the gas laser, provides low power consumption, but it has a shorter service life.
In operation of the bar-code reader employing a semiconductor laser, various methods have been tried to prolong the effective service life of the laser light source.
In an example of a bar-code reader, the laser light source is turned off when the bar-code 101 is not read for a predetermined period and it is reactivated when the bar-code is read again. Means in which an operator controls a switch to activate and cut off a laser light source is used as the reactivating means.
However, such a method has a disadvantage in that because an operator must operate the laser light source to light it on, the good reading operability is not obtained.
As another method, it has been considered to prepare a different light source such as LED in the bar-code reader, in addition to the laser light source. In this method, it is investigated whether the sensor detects light reflected on an approaching article, with the LED emitting. Activating or deactivating the laser light source is controlled based on the presence or absence of the reflected light.
However, the position where the LED is mounted on the bar-code reader is limited. Even if the semiconductor laser is turned off, the LED continues its lighting operation, thus consuming its electric power. The continuous lighting shortens the operational life of the LED, thus causing replacing frequently LEDs. In consideration of such a problem, there is a possibility that employing the LED leads to the higher cost than that of the general bar-code reader.
As a further different method, Japanese Patent Laid-open Publication (TOKKAIHEI) No. 5-55555 discloses the following art.
Referring to the art disclosed in No. 5-55555, as an article or an operator's hand approaches the reading window in a bar-code reading operation, the light amount of an external light (hereinafter referred to as ambient light) around the reader changes. For that reason, a sensor which detects a change in the ambient light is arranged to the bar-code reader so that the lighting of the laser light source is controlled according to the detection result of the ambient light change.
FIG. 26 is a block diagram used for explaining an art disclosed in the above-mentioned Japanese Patent Laid-open Publication No. 5-55555. Referring to FIG. 26, numeral 111 represents an ambient light detector which detects ambient light. The ambient light detector detects ambient light and then outputs an electrical signal corresponding to the light amount thereof.
Numeral 112 represents an amplifier which amplifies an electrical signal output from the ambient light detector. Of signals output from the amplifier 112, the signal (1) representing an ambient light level is input directly to the comparator 113. The integrator 114 slopes the output (2) of the amplifier 112 and then inputs the result to the voltage divider 115. The voltage divider 115 divides the input signal in a voltage ratio and then inputs the resultant output signal as a slice level to the comparator 113.
For example, the ambient light detector 111 detects the ambient light level signal (1), as shown in FIG. 27, while the integrator 114 and the voltage divider 115 set the slice level as shown with the signal (2). These signals (1) and (2) are input to the comparator 113.
The comparator 113 compares the ambient light level signal (1) with the slice level signal (2) and then outputs a switch signal according to the comparison result. In other words, when the ambient light level, for example, is less than the slice level, the switch signal (3) shown in FIG. 27 is output. The switch signal is continuously output for a period during which the ambient light level is smaller than that of the slice level.
The semiconductor laser is controlled so as to be lighted on during which the switch signal is in on state.
When an article is arranged over the reading window to read the bar-code, the ambient light detector detects an ambient light with an area smaller than that of the ambient light prior to the bar-code reading operation. In this art, when the detected ambient light becomes smaller, it is judged that the bar-code reading operation is being performed. Thus the semiconductor laser is emitted only for the reading period.
In the art shown in FIG. 26, the above-mentioned configuration can perform an automatic lighting control of the laser light source over the reading operation to control lighting of the laser light source according to a change in level of ambient light.
However, the above-mentioned art has disadvantages as follows:
The bar-code readers are arranged in various modes and in various external environments.
For example, the bar-code reader may be installed with the reading surface upward or horizontally. Hence, even if the bar-code reader is installed under the same condition regarding external environment such as the level of ambient light, the ambient light level and the degree of ambient light level change are detected differently, with the different installation mode.
The bar-code reader may be installed in a place where the amount of ambient light is small or a very bright place where sunlight falls directly. Moreover it is considered that the bar-code reader also may be installed in a place where a change in ambient light is large or a place where a change in ambient light is small.
It may be considered that even if the same operator reads the same article, an installation environment of the bar-code reader does not allow detecting a change of ambient light level in the case where the same ambient light detection mode (including the slice level setting mode to detect a change in ambient light) exists completely. With different operators, there is the difference in reading operation among individuals.
That is, the distance between an article arranged over the reading window and the ambient light detector depends on the operator's reading operation. As a result, a change in amount of ambient light level is detected differently.
In other words, when the ambient light level is detected as the signal (1) shown in FIG. 28(a) and the slice level is set as the signal (2), provided that a change in ambient light level signal (1) varies at small value, the ambient light level signal (1) is not lowered less than the slice level signal (2). Hence the ambient level signal is not be detected (refer to (a1) and (a2) shown in FIG. 28(a)). In this case, in spite of a completion of the reading operation, the semiconductor laser is not emitted.
As shown in FIG. 28(b), when the level of an input ambient light is very close to the lower limit level, the difference between the ambient light level signal (1) and the slice level signal (2) is very small. In this case, if noises are superposed on the signal (1) or (2), a change in ambient light is erroneously detected.
For example, as shown with the broken lines (c1) and (c2) in FIG. 28(c), noise may be superposed on the ambient light level signal (1). In other words, when noise is superposed on the ambient light level signal (1) as shown with the broken line (c1), it is judged that the ambient light level signal is less than the slice level due to the effect of the noise even if the ambient light level signal is not less than the slice level.
On the contrary, when noise may be superposed on the ambient light level signal (1) as shown with the broken line (c2), it is judged that the ambient light level signal is more than the slice level due to the effect of the noise even if the ambient light level signal is less than the slice level.
In such a case, the semiconductor laser is not emitted on or off desirably. After all, there is a problem in that the automatic lighting control of a semiconductor laser cannot be handled well.